Electrostatic discharge (ESD) is a momentary and sudden electric current that flows when an excess of electric charge, stored on an electrically insulated structure, finds a path to a structure at a different electrical potential, such as ground. ESD is particularly a serious concern with microelectronic devices. The integrated circuits (IC) in these devices are made from semiconductor materials, such as silicon, and insulating materials, such as silicon dioxide, which can break down when exposed to high voltages.
ESD stress occurs in two modes: positive mode ESD and negative mode ESD. Positive mode ESD occurs when there is positive ESD stress at voltage supply (VDD) and ground line is at GND (ground) or when ground line is at VDD and there is negative ESD stress at GND. Negative mode ESD occurs when there is negative ESD stress at VDD and ground line is at GND or when ground line is at VDD and there is positive ESD stress at ground line.
Conventional ESD protection may be integrated onto chips using two types of snapback MOSFET-based strategies: gate-grounded NMOSFET (GGNMOS) and gate-tied to VDD PMOSFET (GVPMOS). These snapback strategies trigger the snapback mechanism to conduct the large amount of ESD current for ESD protection. Both snapback strategies use a RC-triggered power clamp to protect the whole chip from ESD events. These snapback strategies are effective for positive mode ESD stress where the conventional RC-triggered power clamps are fully on during positive mode ESD stress. However, these RC-triggered power clamps typically turn on only weakly and perform poorly during negative mode ESD stress events. Designing to suppress negative mode ESD stress is a major design concern for silicon on insulator (SOI) technology, with floating-body devices.
Another strategy for negative mode ESD protection is to add a parasitic diode into the circuit. However, such parasitic diodes occupy valuable silicon area on a chip and are not optimized for ESD stress. Moreover, parasitic diodes are often not found in floating-body devices.